Sphingosine-1-phosphate (S1P) is part of the sphingomyelin biosynthetic pathway and is known to affect multiple biological processes. S1P is formed through phosphorylation of sphingosine by sphingosine kinases (SK1 and SK2) and it is degraded through cleavage by sphingosine lyase to form palmitaldehyde and phosphoethanolamine or through dephosphorylation by phospholipid phosphatases. It is present at high levels (˜500 nM) in serum, and it is found in most tissues. It can be synthesized in a wide variety of cells in response to several stimuli, which include cytokines, growth factors and G protein-coupled receptor (GPCR) ligands. The GPCRs that bind S1P (currently known as the S1P receptors S1P1-5), couple through pertusis toxin sensitive (Gi) pathways as well as pertusis toxin insensitive pathways to stimulate a variety of processes. The individual receptors of the S1P family are both tissue and response specific and therefore are attractive as therapeutic targets.
S1P evokes many responses from cells and tissues. In particular, S1P has been shown to be an agonist at all five GPCRs, S1P1 (Edg-1), S1P2 (Edg-5), S1P3 (Edg-3), S1P4 (Edg-6) and S1P5 (Edg-8). The action of S1P at the S1P receptors has been linked to resistance to apoptosis, changes in cellular morphology, cell migration, growth, differentiation, cell division, angiogenesis and modulation of the immune system via alterations of lymphocyte trafficking. Therefore, S1P receptors are targets for therapy of, for example, neoplastic diseases, autoimmune disorders and tissue rejection in transplantation. These receptors also share 50-55% amino acid identity with three other lysophospholipid receptors, LPA1, LPA2, and LPA3 of the structurally related lysophosphatidic acid (LPA).
GPCRs are excellent drug targets with numerous examples of marketed drugs across multiple disease areas. GPCRs are cell surface receptors that bind hormones on the extracellular surface of the cell and transduce a signal across the cellular membrane to the inside of the cell. The internal signal is amplified through interaction with G proteins which in turn interact with various second messenger pathways. This transduction pathway is manifested in downstream cellular responses that include cytoskeletal changes, cell motility, proliferation, apoptosis, secretion and regulation of protein expression, to name a few. S1P receptors make good drug targets because individual receptors are expressed in different tissues and signal through different pathways, making the individual receptors both tissue and response specific. Tissue specificity of the S1P receptors is desirable because development of an agonist or antagonist selective for one receptor localizes the cellular response to tissues containing that receptor, limiting unwanted side effects. Response specificity of the S1P receptors is also of importance because it allows for the development of agonists or antagonists that initiate or suppress certain cellular responses without affecting other responses. For example, the response specificity of the S1P receptors could allow for an S1P mimetic that initiates platelet aggregation without affecting cell morphology.
The physiologic implications of stimulating individual S1P receptors are largely unknown due in part to a lack of receptor type selective ligands. Isolation and characterization of S1P analogs that have potent agonist or antagonist activity for S1P receptors have been limited.
S1P1 for example is widely expressed, and the knockout causes embryonic lethality due to large vessel rupture. Adoptive cell transfer experiments using lymphocytes from S1P1 knockout mice have shown that S1P1 deficient lymphocytes sequester to secondary lymph organs. Conversely, T cells overexpressing S1P1 partition preferentially into the blood compartment rather than secondary lymph organs. These experiments provide evidence that S1P1 is the main sphingosine receptor involved in lymphocyte homing and trafficking to secondary lymphoid compartments
Currently, there is a need for novel, potent, and selective agents, which are agonists or antagonists of the individual receptors of the S1P receptor family in order to address unmet medical needs associated with agonism or antagonism of the individual receptors of the S1P receptor family.